1. Field of the Invention
The present invention relates to a honing method and a honing apparatus for grinding the inner surface of a workpiece carried on a machining line by rotating a honing head having hones relative to the workpiece while axially moving the honing head.
2. Description of the Related Art
A workpiece having a cylindrical inner surface, e.g., the cylinder bore of a cylinder block is an important region that determines engine performance. For this reason, the cylinder bore is required to have high shape accuracy and good surface profile. As the shape accuracy, it is required to make the roundness and cylindricity of the cylinder bore as high as possible in order to reduce piston sliding resistance. As for the surface profile, it is desirable to ensure enough roughness to hold oil and expose graphite, serving as a fixed lubricant, to the surface of the bore as much as possible in order to decrease the seizure (scuff) of the piston.
To satisfy these conditions, a honing head 5 having hones 3 is inserted into th inner peripheral surface of the cylinder bore 1, rotated and axially moved to conduct grinding, thereby executing the honing of the cylinder bore 1 as shown in FIG. 8. The honing head 5 has a plurality of hones 3 in a circumferential direction and the honing head 5 hones the inner peripheral surface of the cylinder bore 1 while pressing the hones 3 radially outward, i.e., toward the bore inner peripheral surface of the cylinder bore 1 with a predetermined expansion pressure.
In the above-mentioned honing, it is necessary to secure a certain amount of machining allowances so as to remove roughness generated by boring, a preprocessing to the honing. Normally, therefore, with a view of minimizing honing time, coarse honing having high grinding efficiency is conducted to most of the machining allowance and, after the coarse honing, finishing honing having low grinding efficiency is conducted to improve the shape accuracy and the surface profile.
During the coarse honing, in order to secure an amount of machining allowances as described above, the cylinder bore 1 is largely deformed by the expansion pressure which is a pressure with which the hones 3 are pressed against the cylinder bore 1. FIG. 9A shows the deformation state of the cylinder bore 1 during the coarse honing. According to FIG. 9A, the cylinder bore 1 is deformed so as to largely widen the upper opening side of the cylinder bore 1 radially outward and to decrease the inside diameter of the lower portion of the cylinder bore 1.
If the expansion pressure of the hones 3 is released and the workpiece is left as it is in this deformation state, a function of returning the inner peripheral surface of the pressed cylinder bore 1 to the central side, i.e., springback indicated by arrows S occurs. As a result, the shape of the cylinder bore 1 becomes, for example, that shown in FIG. 9B.
If the subsequent finishing honing is carried out continuously to the coarse honing while the cylinder bore 1 is being deformed as shown in FIG. 9A, it becomes disadvantageously difficult to highly accurately secure the shape of the cylinder bore 1 due to the influence of the springback.
In addition, to secure the shape accuracy, such measures as to decrease the expansion speed of the hones during the coarse honing to thereby decrease the springback of the workpiece or to lengthen finishing honing time may be considered. However, these measures disadvantageously lengthen machining time and push up manufacturing cost.